In a heater of this kind, it is known to provide a resistance heating element of coil or ribbon form having a temperature coefficient of resistance which is low relative to that of the at least one infra-red lamp and connect this in series with the infra-red lamp or lamps at least in some conditions of operation of the heater. Such a resistance element contributes to the output power of the heater, particularly enabling low output power to be obtained from the heater at low energy settings of the cyclic energy regulator and also acting as ballast for the infra-red lamp or lamps to prevent excessively high current flow on energising the heater.